It is known, for example see U.S. Pat. No. 4,720,977, to sense the pressure of air supplied to a turbo-charged diesel engine, and to use the sensed air pressure in controlling the timing of fuel delivery to the engine. For example, if the air pressure differs from a predetermined steady state air pressure as occurs during acceleration, the timing of fuel delivery can be changed to alter the combustion characteristics so that the energy contained in the exhaust gases is increased causing the speed of rotation of the turbine of the turbo-charger to rise. As a result, the pressure of air supplied to the engine is increased, thereby allowing more fuel to be burned in the engine and hence permitting an increase the torque of the engine.
By monitoring the air pressure supplied to the engine and using the measured air pressure in controlling the quantity of fuel delivered to the engine, the increase in output torque occurs earlier than would otherwise be the case thus "turbo lag" is reduced.
The amount of boost air supplied to the engine limits the maximum quantity of fuel supplied and this is important in controlling smoke emitted by the engine.
Clearly, if the air pressure sensor fails, such a system would not operate correctly. It is known to control the engine using a fixed default air pressure value in such circumstances but this is disadvantageous in that such a value will not be ideal and hence the power output of the engine may be reduced to reduce smoke emissions.
It is an object of the invention to provide a control method in which the disadvantageous effects set out hereinbefore are reduced.
According to the present invention there is provided a method of controlling a turbocharged engine comprising the steps of:
(a) using at least one engine parameter to derive a predicted transient air pressure; PA1 (b) sensing an actual air pressure using a pressure sensor; and PA1 (c) comparing the predicted transient air pressure with the actual air pressure to determine a predicted boost pressure error.
The method may comprise the additional step of determining whether the predicted boost pressure error falls within an acceptable range. The result of this additional step can be used to provide an indication of whether the air pressure sensor is functioning correctly--if the predicted transient air pressure is significantly different from the actual air pressure as may be the case if the pressure sensor has failed, then the predicted boost pressure error will be high.
If it is determined that the air pressure sensor has failed, then conveniently the predicted transient air pressure is used in controlling the quantity of fuel delivered to the engine rather than using a fixed default value as in the prior arrangements. As a result, the output power of the engine can be maintained rather than reduced as in the prior arrangements.
The method conveniently includes the step of determining, using the predicted boost pressure error, whether the engine is in a transient condition. If it is determined that the engine is in a transient condition, then an advance offset may be derived using (i) either the actual air pressure or the predicted transient air pressure and (ii) a predicted steady state air pressure derived using at least one engine parameter, the advance offset being used to determine the timing of fuel delivery to the engine.
The invention will further be described, by way of example, with reference to the accompanying drawings, in which: